There are various kinds of transmissions of a bicycle. Among the transmissions, generally, there are a type (an exterior transmission) which provides multistage sprockets in any one of a crank shaft and a rear axle or on both coaxes and changes gears by moving a chain between the sprockets using a derailleur, and a type (an interior transmission) which changes gears by exchanging a gear provided inside a rear hub of a rear wheel serving as a drive wheel.
Although the exterior transmission has a simple structure and is lightweight, it becomes a cause of wear on the sprocket and the chain and also a cause of loosening of the chain. Meanwhile, since the interior transmission has an advantage in that protection against dust and waterproofness are excellent and maintenance is free, the interior transmission is widely used for urban cycles.
However, in an electric power-assisted bicycle in which an electric motor adds supplementary force to a human power drive system, there has been an electric power-assisted bicycle which includes a motor, a speed reducer, and a transmission mechanism in the rear hub of the rear wheel serving as the drive wheel of the bicycle.
A so-called rear hub motor type electric power-assisted bicycle provided with a driving motor in the rear hub can use any one of the exterior transmission and the interior transmission when being combined with the transmission.
When using the exterior transmission, since the hub is mainly constituted by the motor and the speed reducer, the structure thereof is simplified, but there is a problem of maintenance due to the exterior transmission as mentioned above. Meanwhile, when using the interior transmission, since the structure of the hub is constituted by the motor, the speed reducer, and the transmission mechanism, the structure of the hub itself is complicated, but since there is an advantage owing to the interior transmission as mentioned above, it is advantageous.
At the present time, the electric power-assisted bicycle has been mainly used in urban cycles, and has mostly adopted the interior transmission. Thus, it also is considered preferable that the rear hub motor type electric power-assisted bicycle adopt the interior transmission.
As the structure of the electric power-assisted bicycle including such an interior transmission, for example, there are structures described in JP Patent publication 9-58568A and JP Patent publication 2000-043780A.
In such a structure, an electric motor (motor), a speed reduction mechanism, and a transmission mechanism are placed in the rear hub. As a driving mechanism using the electric motor, a driving electric motor and a speed reduction mechanism of a driving system that reduces the number of revolutions of the electric motor are provided. Furthermore, as an input mechanism of the human power, an inputting sprocket is provided in an axle of a drive wheel, and the transmission mechanism and the speed reduction mechanism are sequentially placed toward an outer circumference from the axle.
The rear hub includes a rotation casing and a fixed casing, the input mechanism of the human power system is placed on the rotation casing side, and the driving mechanism using the electric motor is mainly placed on the fixed casing side.
The human power applied by a pedal is transmitted to the sprocket of the drive wheel by a chain, is transmitted to the speed reduction mechanism of the human power system after being shifted by the transmission mechanism, and rotates the drive wheel through the rotation casing. Furthermore, driving force due to the electric motor is decelerated by a speed reduction mechanism of an electric power system provided separately from the speed reduction mechanism of the human power system mentioned above, and then, the human power driving force is combined with the electric driving force in the rotation casing and is transmitted to the drive wheel.
At this time, the driving force due to the human power converted into an electric signal, and an electric signal of a running speed from a speed sensor are input to a control section included in the electric power-assisted bicycle, and the control section outputs a driving signal based on a predetermined condition to control the electric motor.
However, in such a structure, the electric motor is placed in a position which deviates from an axial center of the axle. For this reason, there is a problem in that an outer diameter of the hub is increased. When the outer diameter of the hub is large, a weight balance tends to worsen.
At this point, according to the structure described in JP Patent publication 2002-293285A and JP Patent publication 2003-160089 A, since there is a configuration in which the speed reduction mechanism, the electric motor, and the transmission mechanism are added on the axle, an increase in size of the hub can be suppressed.
That is, in such a structure, the speed reduction mechanism, the electric motor and the transmission mechanism are added in the rear hub serving as the rear wheel of the drive wheel in a vehicle width direction, and the human power applied by the pedal is transmitted to the sprocket of the rear wheel via the chain, is shifted by the transmission mechanism via a one-way clutch, and then rotates the drive wheel through the rotation casing. Furthermore, the driving force due to the electric motor is decelerated by a speed reduction mechanism constituted by a planetary gear mechanism provided around the axle, and then the human power driving force is combined with the electric driving force in the rotation casing and the combined force is transmitted to the drive wheel.
However, according to the structure described in JP Patent publication 2002-293285A and JP Patent publication 2003-160089 A, the output from the electric motor is transmitted to a hub case (rotation casing) via the speed reduction mechanism, and the driving force from the pedal is transmitted to the hub case (rotation casing) via the transmission mechanism.
For this reason, three mechanisms of the speed reduction mechanism, the electric motor and the transmission mechanism arranged in a row along the axial center of the axle each have an independent structure. That is, the transfer mechanism of the human power driving force and the transfer mechanism of the electric driving force are individually interposed in each different route.
Since a space of the drive wheel in the axle direction is limited, in such a structure, the space of the transmission mechanism is reduced, and thus it is difficult to increase the number of speeds.